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Slow-spreading submarine ridges in the South Atlantic as a significant oceanic iron source


Low levels of the micronutrient iron limit primary production and nitrogen fixation in large areas of the global ocean. The location and magnitude of oceanic iron sources remain uncertain, however, owing to a scarcity of data, particularly in the deep ocean1. Although deep-sea hydrothermal vents along fast-spreading ridges have been identified as important contributors to the oceanic iron inventory2, slow-spreading ridges, which contribute more than half of the submarine ridge-crest environment, are assumed to be less significant and remain relatively unexplored2. Here, we present measurements of dissolved iron and manganese concentrations along a full-depth section in the South Atlantic Ocean, running from offshore of Brazil to Namibia. We detect a large dissolved iron- and manganese-rich plume over the slow-spreading southern Mid-Atlantic Ridge. Using previously collected measurements of helium-3 concentrations—a tracer of hydrothermal activity—we calculate the ratio of dissolved iron to hydrothermal helium in the plume waters and find that it is 80-fold higher than that reported for plume waters emanating from faster-spreading ridges in the southeastern Pacific3. Only the application of a higher ratio in global ocean model simulations yields iron fluxes from these slow-spreading submarine ridges that are in line with our observations. We suggest that global iron contributions from hydrothermal vents are significantly higher than previously thought, owing to a greater contribution from slow-spreading regions.

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Figure 1: A zonal section of dFe and dMn in the South Atlantic.
Figure 2: Vertical profiles from the vicinity of the MAR of the South Atlantic Ocean and comparison with 3He distributions.

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We are indebted to the captain and crew of the RV Knorr for their considerable efforts in the sampling of this zonal section. We also thank P. Lam, C. Hammerschmidt and A. Cox for assistance at sea, S. Birdwhistell for assistance in the WHOI inductively coupled plasma mass spectrometry facility and C. German, C. Devey and G. Henderson for conversations. We thank the GEOTRACES community and intercalibration programme. We thank J. Resing for comments. This research was financially supported by the US NSF-Chemical Oceanography programme (OCE-0452883, OCE-0752291, OCE-0928414, OCE-1031271 and OCE-1233261) and the Gordon and Betty Moore Foundation Grant @2724.

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The ocean section sampling expedition plan was designed by M.A.S. and C.H.L. The field sampling programme was orchestrated and implemented by T.J.G., M.A.S., C.H.L. and A.E.N. (also see Acknowledgements). Fe and Mn analyses were made by A.E.N. Optimum multiparameter analysis and gridding analyses were conducted by W.J.J. A.T. analysed the NEMO-PISCES model output comparison. Data analysis and interpretation was conducted by M.A.S., W.J.J., A.T., A.E.N. and C.H.L. The manuscript was written by M.A.S., W.J.J., A.T., A.E.N. and C.H.L.

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Correspondence to Mak A. Saito or William J. Jenkins.

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The authors declare no competing financial interests.

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Saito, M., Noble, A., Tagliabue, A. et al. Slow-spreading submarine ridges in the South Atlantic as a significant oceanic iron source. Nature Geosci 6, 775–779 (2013).

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